clk: sunxi-ng: enable SUNXI_CCU_MP for PRCM

The newly added PRCM CCU driver uses SUNXI_CCU_MP_WITH_MUX_GATE, which causes
a link error when no other driver enables SUNXI_CCU_MP:

drivers/clk/built-in.o:(.data+0x5c8c8): undefined reference to `ccu_mp_ops'

This adds an explicit 'select' statement for it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>